Casein kinase 1 (CK1) is a family of evolutionarily conserved serine/threonine kinases including seven known members in vertebrates (CK1α, -β, -γ1, -γ2, -γ3, -δ and -ε). The CK1s contain a typical kinase domain followed by a C-terminal tail region, which has been implicated in the regulation of CK1 localization, substrate selectivity and kinase activity. Myriad proteins have been found to be phosphorylated by CK1s, which are involved in a wide range of cellular functions including vesicular trafficking, DNA damage repair, cell cycle progression, cytokinesis and circadian rhythms (reviewed by Gross and Anderson (1998); Vielhaber and Virshup (2001); Knippschild et al. (2005)). Moreover, CK1 family members (-α, -δ/ε and -γ) modulate the activities of major signaling pathways (for example, Wnt and Shh) through several mechanisms (Peters et al., 1999; Liu et al., 2002; Price and Kalderon, 2002; Davidson et al., 2005; Zeng et al., 2005 and reviewed by Price (2006)).
In mammals seven CK1 isoforms, namely CK1α, β, γ1-3, δ and ε, and several splice variants have been described. They all contain a highly conserved kinase domain, a short N-terminal domain of 6 to 76 amino acids and a highly variable C-terminal domain of 24 to more than 200 amino acids. The constitutive phosphotransferase activity of CK1 isoforms is tightly controlled by several mechanisms. For example, the closely related isoforms CK1δ and ε, which share a 98% identity at the amino acid level in their catalytic domain, are regulated by autophosphorylation, dephosphorylation and proteolytic cleavage. Members of the CK1 family are found in the nucleus, the cytoplasm and in the plasma membrane. By phosphorylating many different substrates bearing either a canonical or non-canonical consensus sequence they modulate the activity of key regulator proteins involved in many cellular processes such as cell differentiation, cell proliferation, apoptosis, circadian rhythm, chromosome segregation, and vesicle transport.